The present invention relates to devices for use in measuring the characteristics of microwave circuits.
Microwave circuits require special testing devices because of their unusual properties. Therefore, devices generally referred to as network analyzers have been developed to measure various microwave circuit characteristics such as reflection, transmission, gain, impedance and phase as a function of frequency and time.
These devices operate by establishing a test frequency, connecting to a standard circuit for calibrating the analyzer and then, after disconnecting the standard circuit, connecting to a device under test (DUT) for making the desired measurements.
However, present network analyzers have been subject to problems in use due to the system they employ for connecting to the calibration circuits and the circuits to be tested. Ordinarily they transmit a microwave test signal to the circuit to be tested along a transmission (coaxial or waveguide) line. The transmission line must be physically manipulated to make the actual connection with the calibration circuits and then manipulated again to connect and/or disconnect each circuit to be tested. These physical manipulations alter the microwave characteristics of the transmission line, thereby affecting the test signal and resulting in significant measurement errors.
These errors have been generated because the typical manner in which the measurements are taken is to generate a microwave signal at a source in the chassis of the measuring equipment and to provide also at the source the appropriate mixers and intermediate frequency (IF) circuits for developing the information containing signals. Therefore, the RF high frequency signals are transmitted over appropriate transmission lines to the DUT and the reflections are transmitted back over the transmission line from the DUT to the mixers. As a result, the calibration accomplished with the appropriate standards affixed to the cables includes the cables as part of the calibration. Thus even the slightest movement of the cables in removing the standard and attaching the DUT renders the calibrations at least somewhat erroneous if not invalid and in turn renders DUT measurements inaccurate to a like extent.
The present invention overcomes the aforementioned problems by providing a system in which manipulations of the transmission line between the network analyzer and the cirucit to be tested and other related changes in environmental conditions do not appreciably affect the measurement being made.